The present invention relates to a method and apparatus for charge stripping analyte ions.
Ionisation techniques, such as Electrospray Ionisation (ESI), facilitate the analysis of high molecular weight species by generating multiply charged ions. As these ions have relatively high charge states, when their mass to charge ratios are detected they fall into the low mass to charge ratio range of the spectrum. If a sample being analysed contains a mixture of ions of relatively high charge states then this can cause spectral congestion, because the mass to charge ratios of the ions become bunched at the low mass to charge ratio range of the spectrum. Even with extremely high mass resolving power or other separation methods this may result in ambiguities in assignments due to peak overlap and unknown charge states.
The charge carriers on the analyte ions in ESI techniques may be a mixture of protons, metal cations or cations from the electrospray solution, as well as counter ions.
It is known to reduce the charge states of analyte ions so as to reduce the above-described spectral congestion. As the charge states of the analyte ions are reduced, their mass to charge ratios are increased, thus spreading out the detected peaks along the mass to charge ratio scale.
For biopolymer ions, such as for example, native proteins and pegylated polymers, the ultimate spectral simplicity would be obtained if the charge state could be reduced to a relatively low value (i.e. to +/−1 or +/−2). The generation of singly charged ions is also beneficial for the study of non-covalently bound protein complexes. However, to the inventors' knowledge such complexes have not been observed before as singly charged ions.
It is known to perform ion-neutral reactions between analyte ion and superbase molecules in order to charge reduce the analyte ions. Such techniques may be used either in-source or under vacuum conditions. However, this technique may be undesirable due to the corrosive effects and toxicity of the superbases.
It is also known to employ in-source ion-ion reactions between analyte ions and reagent ions for the purpose of charge reducing the analyte ions. For example, reagent anions may be generated from a glow discharge device or via photoionisation processes. Such techniques show promise, although it is not known if the charge reduction techniques actually generate significantly higher mass to charge ratio ions, since ions having a mass to charge ratio beyond around 18000 (for a charge state of 2) are not observed at the detector.
In-vacuum ion-ion reactions between analyte ions and ETD reagents are also known. These techniques have demonstrated the ability to generate charge reduced ions via electron transfer and/or charge transfer to radical anions, as well as the observation of signature c and z type fragment ions. However, the published data has not revealed charge reduced ions of single charge state at relatively high mass to charge ratios.
It is desired to provide an improved method and apparatus for charge stripping analyte ions, an improved method and apparatus for performing mass spectrometry, and an improved method and apparatus for performing ion mobility spectrometry.